Why do people hate when people are "negative"?

This group is really the only group I have participated in where people don't seem averse to being "negative" or critical. Normally when you criticize something, or speak too negatively about something, people start getting offended, and tell you to stop being so negative. Why is this? I don't have this instinct that makes people upset at negativity. If someone has something negative to say, I am usually interested to hear it.

Negativity is contagious, so people instinctively protect themselves against negative agents, because negativity is not just about holding a critical view of something or opposing someone's opinion, it can induce functional brain changes, affect the way you think and lead to myriad mental/neurological issues. People can tolerate negativity but usually don't feel comfortable around other people who are negative by nature. A bunch of kids speaking negatively about a fashion brand or a gadget causes others from the group to develop a similar view but a kid overwhelming others with negative rants will get ostracized.